Classifying Water Treatment Units: Operations and Processes for Smart Cities

Water treatment is the backbone of every smart city’s public health infrastructure. Before water reaches your tap or leaves as treated wastewater, it undergoes a series of carefully designed treatment steps. These steps fall into two fundamental categories: unit operations and unit processes. Understanding how these work together-from screening out debris to settling particles and chemically treating contaminants-reveals the engineering precision behind clean water delivery in modern urban systems.

Unit operations vs. unit processes: the core division
The first line of defense: screening in water treatment
A deep dive into fine screening technologies
Coarse screens: handling the bulk solids
Micro screens and the sedimentation process
Designing efficient sedimentation tanks
The four types of particle settling explained
Coagulation and filtration in water purification

Unit operations vs. unit processes: the core division

Treatment methods relying on physical forces are called unit operations. These include screening, sedimentation, and filtration-processes where contaminants are removed through mechanical or gravitational action without altering their chemical composition. In contrast, unit processes involve chemical or biological activities that transform contaminants to facilitate their removal.

Common unit operations include screening (removing large solids), grit removal (settling inert particles), and sedimentation (gravity-based separation). Unit processes, on the other hand, include coagulation using chemicals like alum or ferric chloride, disinfection through chlorination, and biological treatment such as the Activated Sludge Process (ASP). Most treatment plants combine multiple unit operations and processes in sequence to achieve comprehensive water purification.

The first line of defense: screening in water treatment

Screening is typically the very first step at any water or wastewater treatment facility. Its primary purpose is to remove large debris such as rags, plastics, paper, metals, and other solid materials that could clog or damage downstream equipment. Without effective screening, pumps can jam, pipes can clog, and treatment efficiency drops significantly.

Screens are classified based on the size of their openings into three main categories: coarse screens (6-150 mm openings), fine screens (less than 6 mm openings), and micro screens (openings as small as 10-35 µm). Each type serves a specific purpose in progressively filtering out smaller and smaller particles as water moves through the treatment train.

A deep dive into fine screening technologies

Fine screens have openings smaller than 6 mm and are constructed from wire cloth, wedge wire, or perforated plates. They capture smaller debris that passes through coarse screens, including food particles, hair, and fine organic matter. Fine screening significantly improves the performance of subsequent biological treatment processes by reducing the solids load.

Three common types of fine screens dominate the industry:

Static wedge wire screens feature a curved or flat panel with closely spaced wedge-shaped wires. Water flows over the screen surface while solids are retained and eventually removed. These screens typically have openings between 0.2 and 1.2 mm and are designed for flow rates of 400 to 1,200 litres per square metre per minute. They require substantial floor space and periodic manual or automatic cleaning.

Rotary drum screens consist of a cylindrical drum with screening media mounted on its surface. As the drum rotates, wastewater enters and passes through the screen while solids are captured inside or outside (depending on design). The captured material is conveyed to a collection point for disposal. These screens handle varying flow conditions effectively.

Step-type screens use alternating fixed and movable plates arranged across the channel width. The stepping motion lifts captured solids upward while allowing water to pass through. This self-cleaning mechanism makes them suitable for continuous operation with minimal maintenance.

Coarse screens: handling the bulk solids

Coarse screens, also known as bar racks, are positioned at the headworks of treatment plants to intercept large floating and suspended objects. Coarse screens have openings ranging from 6 to 150 mm and consist of parallel bars, rods, or perforated plates with circular or rectangular apertures.

Based on cleaning method, coarse screens fall into two categories:

Hand-cleaned screens are manually raked to remove accumulated debris. These are typically used in smaller facilities, as standby units during high-flow periods, or when mechanical systems are under repair or during power failures.

Mechanically cleaned screens are more common in larger plants and include several subtypes. Chain-driven screens use automatic chains to rake debris from either upstream or downstream directions. Reciprocating rake screens (also called climber screens) employ a single rake that moves up and down to collect material. Catenary screens are front-return, front-cleaned chain-driven units designed to handle heavy or bulky objects without jamming. Continuous belt screens feature multiple rakes attached to drive chains, providing self-cleaning capability for both fine and coarse solid loads.

Micro screens and the sedimentation process

Micro screens are typically low-speed rotating drum filters lined with filtering fabrics having openings of 10 to 35 µm. They remove very fine suspended particles, including algae in water treatment applications. Wastewater enters the drum, passes through the fabric, and captured solids are removed by water jets or scrapers. Micro screens can sometimes replace primary settling tanks for waters with low colour and colloidal turbidity.

Following screening, sedimentation becomes the primary unit operation for removing suspended solids. Sedimentation uses gravity to separate suspended solids from water. Settling basins are engineered structures where flow velocity is reduced, allowing denser particles to descend to the tank bottom while clarified water exits from the top.

The theoretical basis for sedimentation design comes from Stokes’ law, which describes how spherical particles settle through viscous fluids. Settling velocity is directly proportional to the square of particle diameter and the density difference between particle and fluid, while being inversely proportional to fluid viscosity. This relationship helps engineers calculate the required tank dimensions and detention times for effective solids removal.

Designing efficient sedimentation tanks

Sedimentation tank design centres on two key parameters: settling velocity and overflow rate (also called surface loading). The overflow rate is calculated as the flow rate divided by the tank’s surface area, expressed in units of velocity (metres per second or metres per hour).

Any particle with a settling velocity greater than the overflow rate will theoretically be removed, while particles with lower settling velocities will be removed in proportion to the ratio of their settling velocity to the overflow rate. Design recommendations suggest overflow rates of approximately 9.4 mm/s for quiescent zones, 4.0 mm/s for full-flow basins, and 0.46 mm/s for off-line basins.

A crucial insight in sedimentation tank design is that efficiency depends primarily on surface area, not depth. As long as the forward velocity remains low enough to prevent re-suspension of settled material from the tank floor, increasing depth does not improve removal efficiency. However, practical designs typically use depths of 3-6 metres with retention times of several hours to prevent hydraulic short-circuiting and allow some particle growth through flocculation.

Tanks are commonly built as long rectangular basins (hydraulically more stable for large volumes) or circular clarifiers with central feed and peripheral overflow. Each design includes four functional zones: inlet zone (establishes uniform flow), settling zone (where sedimentation occurs), sludge zone (collects settled material), and outlet zone (discharges clarified water).

The four types of particle settling explained

Not all particles settle the same way. Particle settling behaviour depends on concentration and the tendency of particles to interact. Engineers recognize four distinct settling types:

Type I: Discrete settling occurs in dilute suspensions (typically below 500-600 mg/L total suspended solids). Particles settle independently without interacting with neighbours. Each particle maintains constant size, shape, and settling velocity throughout descent. Grit chambers exemplify discrete settling conditions, and Stokes’ law directly applies to this regime.

Type II: Flocculent settling happens in dilute suspensions where particles can collide and aggregate during settling. As particles flocculate, they grow larger and settle faster. This type typically occurs at solids concentrations below 1,000 mg/L and is characteristic of primary sedimentation tanks (PST) and clariflocculators.

Type III: Zone (hindered) settling occurs at intermediate to high concentrations (roughly 1,200-5,000 mg/L). Particles are close enough that their velocity fields overlap, causing them to settle as a coherent mass or “blanket” rather than individually. A distinct interface forms between the settling sludge and the clear supernatant above. This behaviour is common in secondary settling tanks following activated sludge treatment.

Type IV: Compression settling takes place at very high concentrations where particles are essentially in contact. Further settling occurs only through compaction as upper layers compress lower layers, squeezing out interstitial water. This occurs at the bottom of secondary clarifiers and in sludge thickeners.

Coagulation and filtration in water purification

Many fine particles and colloids cannot be removed by sedimentation alone because their settling velocities are too low. Coagulation uses chemical additives to destabilize these particles so they can aggregate into larger, settleable flocs.

The most common coagulants are metallic salts: aluminum sulfate (alum) and ferric chloride. These positively charged chemicals neutralize the negative surface charge on colloidal particles, allowing them to clump together. Ferric chloride offers a broader effective pH range (4.0-11.0) compared to alum (optimal between pH 6-8) and produces denser, faster-settling floc. Following coagulant addition, gentle mixing during flocculation allows particles to aggregate into larger masses before settling.

Filtration follows sedimentation to remove remaining suspended particles. Two main types serve different applications:

Slow sand filters operate at low flow rates (0.04-0.08 gallons per minute per square foot) and rely heavily on biological action. A layer of biological growth called the schmutzdecke forms on the filter surface and provides much of the treatment. These filters do not require chemical pretreatment and are cleaned by scraping rather than backwashing.

Rapid sand filters operate at much higher rates (2-10 gallons per minute per square foot) and typically follow coagulation and flocculation. Removal occurs through both straining (trapping particles larger than media pores) and adsorption (particles attaching to filter media through electrical or chemical attraction). These filters require periodic backwashing to remove accumulated solids.

What do you think? As smart cities grow and water demands increase, how might emerging technologies like membrane filtration or advanced oxidation complement these traditional treatment methods? And considering climate change impacts on water quality, what additional treatment challenges might urban water systems face in the coming decades?

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Classifying Water Treatment Units: Operations and Processes for Smart Cities

Table of Contents

Unit operations vs. unit processes: the core division 🔗

The first line of defense: screening in water treatment 🔗

A deep dive into fine screening technologies 🔗

Coarse screens: handling the bulk solids 🔗

Micro screens and the sedimentation process 🔗

Designing efficient sedimentation tanks 🔗

The four types of particle settling explained 🔗

Coagulation and filtration in water purification 🔗